This Invention relates to an improved pressure sensitive adhesive for use with adhesive diaper closure tapes and the like, and more specifically, to a pressure sensitive adhesive composition which provides improved performance characteristics when used in a diaper closure system.
There are numerous patents and literature documents that are directed to the use of block copolymers in adhesive compositions which traditionally include a block copolymer such as an ABA block copolymer and a tackifying resin. U.S. Pat. No. 3,427,269 (Davis), which describes the use of an elastomeric based adhesive comprising at least 100 parts of an ABA block copolymer a rosin ester (10-50 parts) and a coumarone-indene resin (25-50 parts). However, as development of these elastomer based compositions has progressed it has been found that the adhesive properties of these block copolymer based adhesive compositions are extremely sensitive to the particular block copolymer(s) used, and the proportion and type of modifiers such as solid or liquid tackifiers, aromatic or aliphatic tackifiers, plasticizers, extender oils, curing agents and the like.
U.S. Pat. No. 3,932,328 (Korpman) describes the use of prior art adhesive based on an ABA block copolymer with a solid tackifying resin and an extender oil. He describes this composition as unsatisfactory as a pressure-sensitive adhesive(PSA) and proposes a specific composition comprising a styrene-isoprene-styrene (SIS) copolymer and specific proportion of an aliphatic solid tackifier and a liquid tackifier (at 25.degree. C.). Korpman alleged that his adhesive had "satisfactory" adhesion and quick-stick for most applications. The prior art patent Korpman was referring to was U.S. Pat. No. 3,239,478 (Harlan) who discloses both PSA and non-PSA ABA block copolymer based adhesives Harlan only exemplifies a PSA formed of 100 parts of a SIS block copolymer (48 percent S), 75 parts of a 2% aromatic extender oil and 125 parts of a tackifying resin of glycerol ester of polymerized rosin.
U.S. Pat. No. 3,954,692 (Downey et al) also addressed the problems with ABA based adhesive compositions, particularly hot melt pressure sensitive adhesive compositions, particularly problems with providing adequate adhesion and strength properties. Downey et al. was particularly concerned with providing an adhesive with not only adequate tack but good peel strength and good shear strength Downey et al. proposed an SIS block copolymer admixed with conventional extender oils, which were stated as unsatisfactory by Korpman, using a specific tackifying resin. The resin exemplified comprised a polymerized admixture of piperylene and 2-methyl-2-butene, which is marketed, e.g., as Wingtack.TM. 95, available from Goodyear Chemical Company.
An alleged improvement, for an adhesive designed to be used with porous substrates (e.g., Kraft paper), on the Downey et al. formulation is proposed by U.S. Pat. No. 4,097,434 (Coker) which describes conventional practice as using extending oils with low aromatic content in formulations in which extending oils are employed. Coker states that these formulations have inadequate holding power to porous substrates and proposes a formulation of an ABA copolymer (only SIS is exemplified) with a tackifying resin (only the Downey et al. Wingtack.TM. 95 is exemplified) with an extending oil with a high aromatic content (above 55%) and a saturates content of less than 15%. The two oils compared were Shellflex.TM.371 (non-invention) and Dutrex.TM.739 (invention). The Shellflex.TM.371 formulations were presented to show formulations with inadequate adhesion properties.
Another variation is proposed in U.S Pat. No. 4,460,364 (Chen et al.) which describes a composition having certain rheological properties that they propose are indicative of ABA based adhesives with tenacious bonding and clean release, where the B block is a polyolefin (only polyethylene-polybutylene exemplified). The SEBS copolymers exemplified are used with a solid tackifying resin and a plasticizing or extending oil. Chen et al. states that these compositions do not sacrifice clean release for tenacity as is stated is typically done in prior art hot melt PSA adhesives. Chen et al. was primarily concerned with an adhesive which could be used in a sanitary napkin without leaving residue adhesive on the undergarment. This rheological approach is also proposed by U.S. Pat. No. 4,719,261 (Bunnelle et al.) who uses the same rheological properties to define a hot melt PSA that allegedly can also be used as an elastic. The specific compositions claimed comprise specific ABA (i.e., SIS) copolymers (45-75%) with both aromatic (2-9%) and aliphatic (25-50%) solid resins.
U.S. Pat. No. 3,935,338 (Robertson et al.) describes an adhesive composition that allegedly has a low enough viscosity to be applied as a hot melt and will have enhanced PSA properties at elevated temperatures. The formulation exemplified as meeting this requirement is an SIS block copolymer (100 parts) with an elastomer block compatible resin (50-200 parts), a reinforcing resin (25-100 parts) and an extending oil (25-100 parts). The patent also discloses compositions excluding the reinforcing resin as counterexample formulations that have inadequate adhesive properties at elevated temperatures.
Korpman has also proposed modifications to his formulations, and approach; U.S. Pat. No. 4,813,947 (use of a ABA(SIS) and a tackifier which is coupled with a specific hot melt landing area); 4,540,415 (use of a SIS a solid tackifier and a phosphatide) and 4,080,348 and 4,136,071 (addition of an SI block copolymer).
Although there exists extensive art on the use of block copolymers in PSA compositions, due to the uncertainty in results and continuing need, the search continues for adhesive formulations with yet improved mixes of properties such as adhesion characteristics, such as quick-stick, tack, shear strength, peel strength and hold; cohesion properties and the ability to maintain these properties at various temperatures and over time. Particularly, there is the continuing need for the identification of adhesive formulations that have a proper mix of adhesive properties such as would make them particularly suitable for use in diaper closure systems. The engineering of diapers is highly specialized. Adhesive closure systems used in this environment ideally should have a particular set of properties. The adhesive tab of the closure system should be designed to readily and quickly attach to the outer polymer film portion of the diaper, which is typically a polyolefin film (e.g., polyethylene or polypropylene), which often has a coating that reduces adhesion thereto. This desire for an adhesive fastening tab with the ability to quickly yet not overly aggressively stick to a treated or untreated polymer film often conflicts with the need for an adhesive bond that has a shear strength such that it will not shift when under stress, as is generated by the user, sufficient yet not overly aggressive peel force resistance, and the ability to allow the fastening tab to be removed and reapplied to the diaper outer film or shell without destruction thereof, which properties are themselves often conflicting. This is complicated by the fact that use of "LAB"(Low Adhesion Backsize) layers such as urethane coatings, often used on diaper frontal tapes, interfere with a fastening tab's adhesive properties.
As such it is a general object of the invention to solve the problems in the diaper closure art by providing diaper fastening tabs with an adhesive formulation directed to solve these problems.
Another object is to provide a diaper closure system using a hot melt PSA with improved field performance characteristics.
Another object is to provide a diaper closure system adhesive formulation showing superior peel strength, quick-stick and shear strength in a diaper system using untreated polyolefin film substrates where the adhesive fastening tab adheres.
Another object is to provide an adhesive formulation having superior peel strength, tackiness, hold and shear strength when the adhesive fastening tab is used to attach to adhesive reduction treated substrates.
These and other objects will be apparent to those skilled in the art by reading the attached specification and by practice of the invention disclosed therein.